[FEA] Add a Spark 3.X Explain only mode to the plugin

[tgravescs]

Is your feature request related to a problem? Please describe.
For Spark versions 3.0+ add support to the plugin to run on the CPU in explain only mode. This would be similar to the explainPotentialGpuPlan api added but instead of user having to modify their code just have a config that says run the plugin in explain only mode and for each of the plans it just dumps out explain output to the logs but still runs everything on the CPU.

This could be used by customers who are evaluating using the plugin but don't have access to GPUs yet and want to see what parts of their query will run on the GPU and perhaps which features aren't yet supported.